Released just this week, the Perpetual Testing Initiative expansion to Portal 2 has already resulted in a success of some kind. No, no. Don't tell me. I'll figure out the appropriate adjective eventually.
More than 35,000 tests have been created by players with in excess of 1.3 million downloads of those tests and Lord only knows how many screamed expletives and dead test subjects. Come to think of it, Valve probably knows that too.
In a related story, Steam has started a sale promotion in celebration of the Perpetual Testing Initiative launch by marking down prices on Portal 2 and fellow Steam Workshop game, Skyrim, as well as a number of Workshop items for Team Fortress 2.
Keep up the testing folks!